The present invention relates to a method and apparatus for adjusting the compression ratio of internal combustion engines, and more specifically to a method and apparatus for adjusting the position of the crankshaft with eccentric crankshaft main bearing supports.
Designs for engines having eccentric crankshaft main bearing supports have been known for some time. In these engines the eccentric main bearings are rotated to adjust the position of the crankshaft's axis of rotation. Poor rotational alignment of the eccentric main bearing supports is a problem for these engines because even small amounts of main bearing misalignment can cause rapid main bearing failure.
Significant forces bear down on the eccentric main bearing supports during operation of the engine. In modem passenger car engines main bearing loads can exceed 50 MPa. The forces exerted on the eccentric main bearing supports are, at times, significantly different from one eccentric main bearing support to the next. For example, in multi-cylinder engines a clockwise torque may be applied on a first eccentric main bearing support from the combustion pressure bearing down on the first piston, connecting rod and crank throw, and a counterclockwise torque may be applied on a second or third eccentric main bearing support from the inertial forces of the second piston and connecting rod pulling up on the second crank throw. As a second example, in a single cylinder engine having two eccentric main bearing supports the torque applied to the crank throw and the resistive torque at the power take-off end of the crankshaft cause uneven loading on the eccentric main bearing supports. These large unequal forces are a problem because they cause the eccentric sections to rotate out of alignment with one another causing rapid failure of the crankshaft main bearings.
In U.S. Pat. No. 887,633, and in German patent DE 3644721 A1 a pinned linkage is show for adjusting the rotational alignment of the eccentric main bearing sections. U.S. Pat. No. 4,738,230 shows dowels extending from each eccentric main bearing support that are fitted into slots located in a slidable bar for adjusting the rotational alignment of the eccentric main bearing supports. U.S. Pat. Nos. 5,572,959 and 5,605,120 show gear teeth extending from eccentric main bearing supports that engage a layshaft with mating gears for adjusting the rotational alignment of the eccentric main bearing supports. U.S. Pat. No. 1,160,940 shows a bail shaped frame that connects adjacent eccentric sections for adjusting the rotational alignment of the eccentric sections. Poor alignment of the main bearings is a significant problem for each of these systems. In addition to poor main bearing alignment, a number of these systems are not mechanically functional for other reasons, are impractical for mass production manufacture and assembly, and/or are not functional for engines having more than two main bearings. For example, U.S. Pat. No. 1,160,940 shows a bail shaped frame that is weakly connected to the eccentrics and that does not have a rigid construction. In addition to not rigidly hold the bearings in alignment, the system is not mechanically functional because the connecting rod does not clear the bail shaped frame. The system is also not functional for engines having more than two main bearings because it is not possible to slide the eccentric main bearing support onto the center crankshaft journal or journals.
A further problem with engines having rotatable eccentric main bearing supports in a fixed engine housing is that the location of the crankshaft rotational axis changes with change of compression ratio, making use of a conventional in-line clutch impossible. Geared power take-off couplings for engines having an adjustable crankshaft rotational axis are shown in the prior art, however a problem with these systems is that heavy structural reinforcing is required to rigidly hold the gear set in alignment. In addition to the problem of added weight, engine housing length is also increased.
German patent DE 3644721 A1 shows a gear set mounted to the free end of one of the eccentric crankshaft main bearing supports. The gear set has an intermediary shaft and an output shaft. The output shaft points generally away from the crankshaft, and has a fixed axis of rotation for all compression ration settings. A problem with the system shown in German patent DE 3644721 A1 is that during periods of high engine torque the end eccentric main bearing support may bend out of alignment, resulting in damage to the crankshaft main bearing. The gear set is also bulky and increases cranktrain friction losses due to the increased number of bearings and gear friction. U.S. Pat. No. 4,738,230 shows a first spur gear mounted on the crankshaft and a second spur gear having an axis of rotation that is concentric with the axis of rotation of the main bearing supports. These gears are too small to carry the torsional loads of the engine. U.S. Pat. No. 4,738,230 also shows a power take-off system having an internal or annular gear set. Heavy and lengthy structural reinforcing is required for holding the ring gear shaft in rigid alignment with the gear mounted on the end of the crankshaft. U.S. Pat. Nos. 5,443,043, 5,572,959 and 5,605,120 show a crankshaft having a fixed axis of rotation and an upper engine that changes position relative to its supporting frame when the compression ratio is changed. While a conventional in-line clutch can be employed with this arrangement, the position of the upper engine is changed when the compression ratio is changed, and the inertial mass of the upper engine prevents rapid adjustment of compression ratio.
A further problem with variable compression ratio engines is that the exhaust valves must be closed early and the intake valves opened late in order to prevent valve to piston strike near top dead center (TDC) of the piston. The short valve overlap period where both valves are open is a problem, because air flow into the engine is restricted causing a loss of engine power. Valve pockets can be formed in the piston to increase valve to piston clearance, however, the pockets add volume to the combustion chamber causing the compression ratio of the engine to be reduced. The base height of the piston can be raised further to compensate for the increase of chamber volume, however increasing the piston height increases the depth of the valve pockets. A significant problem is that the relatively large valve pockets cause increased heat loss from the combustion chamber due to the increased chamber surface area and due to the jagged chamber surface shape. The increased heat loss adversely effects engine fuel economy and power. Camshaft phase shifters such as those used on the Lexus LS 400, and/or cam profile switching devices such as those used on Honda VTech engines can be employed to prevent piston to valve strike, however, in addition to being expensive, these devices may fail to react fast enough in some vehicles that have been aged. Compression ratio may be changed in less than one second, and possibly within a tenth of a second. Failure of the variable valve device to respond at least as quickly as the variable compression ratio devise could result in valve to piston strike, causing major engine failure resulting in a significant warranty cost.
A problem with variable compression ratio mechanisms is that the actuator consumes a significant amount of energy, off-setting the fuel economy benefit of the variable compression ratio. U.S. Pat. No. 5,611,301 issued to Per Gillbrand and Lars Bergsten of Saab Automobile Akliebolag, for example, shows a variable compression ratio mechanism where the entire upper engine moves. A significant amount of power would be consumed to rapidly move the engine and change the compression ratio, off-setting the fuel economy benefit of the variable compression ratio. A central problem with variable compression ratio mechanisms is the power consumed in the process of adjusting the compression ratio.
Primary engine balancing can be accomplished with twin counter rotating balance shafts. A problem with balance shafts, however, is that of added bearing friction and windage, which adversely effects engine efficiency and vehicle fuel economy. Single balance shaft are employed in many single-cylinder motorcycles such as the Honda XR650L, the Kawasaki KLX250R, and the BMW F650. In these engines half of the balance mass on the balance shaft and half of the balance mass is placed on the crank web, however, vibration remains significant due to the moment remaining between the crankshaft and single balance shaft axes of rotation.